A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
EUV radiation sources typically use plasma to generate the radiation. Plasma consists of a collection of free-moving electrons and ions (atoms that have lost electrons). The energy needed to strip electrons from the atoms to make plasma can be of various origins: thermal, electrical, or light (ultraviolet light or intense visible light from a laser).
In addition to EUV radiation, radiation sources used in EUV lithography generate contaminant material that may be harmful for the optics and the working environment in which the lithographic process is carried out. Such is especially the case for EUV sources operating via a plasma produced discharge source, such as a plasma tin source. Such a source typically comprises a pair of electrodes to which a voltage difference can be applied. In addition, an energy beam (for example, a laser beam) may be used to trigger the discharge by producing a vapor by impinging on, for example, one of the electrodes. Accordingly, a discharge will occur between the electrodes, generating plasma, and which causes a so-called pinch in which EUV radiation is produced. More details on the pinch, the laser triggering effect and its application in a source with rotating electrodes may be found in US 2004-010508.
In addition to this radiation, the discharge source typically produces debris particles, among which can be all kinds of microparticles varying in size from atomic to complex particles up to 100 micron droplets, which can be both charged and uncharged.
It is desired to limit the contamination of the optical system that is arranged to condition the beams of radiation coming from an EUV source from this debris. Conventional shielding of the optical system primarily includes a system comprising a high number of closely packet foils aligned parallel to the direction of the light generated by the EUV source. A so-called foil trap, for instance, as disclosed in EP1491963, uses a high number of closely packed foils aligned generally parallel to the direction of the light generated by the EUV source. Contaminant debris, such as micro-particles, nano-particles and ions can be trapped in walls provided by the foil plates. Thus, the foil trap functions as a contamination barrier trapping contaminant material from the source. Due to the arrangement of the platelets, the foil trap is transparent for light, but will capture debris either because it is not traveling parallel to the platelets, or because of a randomized motion caused by a buffer gas. It is desirable to improve the shielding of the radiation system, because some (directed, ballistic) particles may still transmit through the foil trap.
It is also desirable to improve the source characteristics for providing a stable source with an increased output and higher repetition frequencies. The throughput of a lithographic apparatus depends upon the output of the radiation source used, both in terms of intensity and repetition rate. However, increasing the radiation output and/or repetition rate of a discharge source typically leads to more debris production due to the higher temperatures reached and subsequent heat load on the components. In some cases, the components of the source system may fail because they cannot be adequately cooled.